The present invention relates generally to solder reflow assembly techniques and apparatus therefor, more particularly to an apparatus and method for forming solder terminals between a substrate and a device, and rework processes.
A very common technique for joining an electronic element and a supporting substrate is forming solder connections between same. Patents which describes such solder connections and a method of forming these connections are U.S. Pat. Nos. 3,429,040 and 3,495,133. In general solder wettable pads are surrounded by non-solder wettable material, i.e., glass or quartz are provided on the device and also on the substrate. Solder is evaporated through a mask on either the device or substrate and heated to the melting point of the solder which causes it to ball up over the pads into mounds. After the solder balls have solidified, the device is positioned over the substrate with the respective solder pads in alignment, and the assembly heated to again melt or reflow the solder. When the solder balls are remelted and allowed to solidify solder connections are established between the solder wettable pads of the device and substrate. The surface tension of the molten solder will shift the device to correct for minor misalignments. If a device or a connection is defective, the device can be removed and replaced by remelting the solder connections, lifting off the device, and replacing it. When the substrate contains numerous devices, multiple device rework operations may be necessary before all the connections are perfect or to replace devices that fail in use.
It has been noted that repeated remelting of the solder, and exposure to high temperatures, particularly above 300.degree. C. for extended time, as well as a slow rate of cooling from these temperatures causes the formation of intermetallics in solder connections which have a degrading influence. Intermetallics are compounds or alloys of the molecules of the solder and possibly the metals in the pad that have been placed in intimate contact during a process of brazing or soldering. In addition, exposure of the solder terminals to high temperatures causes grain growth within the terminals. Both intermetallics and grain growth degrade the solder connection by making it more brittle and less resistant to stress and strain. Strain is imposed on the solder connections by thermal cycling which normally occurs during use, particulrly when the coefficients of expansion of the materials of the device and substrate are different.
It is conventional to reflow the solder mounds to form a connection by placing the substrate with devices in place on a belt furnace and passing the assembly through the heated environment. The same basic process is used for other solder connections such as I/O pins to a substrate. This exposes the substrate, the device, and the solder bonds to a prolonged heating period, typically on the order of 20 minutes or longer. As the mass of the substrate increased, the non-uniformity of temperature in the substrate increased, the time at elevated temperature increased and the cooling rate decreased. Still further, as the substrate area increases, the number of devices on the substrate increase thereby increasing the probability of more rework operations, each of which requires a device separation and a solder reflow joining operation. Still further, the prolonged heating may damage the devices by degrading the metallurgy and spreading the diffused impurities thus moving the PN junctions and expanding the device impurity regions.